CYTOLOGICAL ASPECTS OF HYBRIDITY 219 



This may be the case even when the genomes of the parents differ in the 

 number of members. Thus in Fx pansy hybrids between Viola arvensis 

 (n = 17) and V. tricolor (n = 13) the number of bivalents varies and there 

 is some steriUty, but later generations are made up of both the parental 

 types and a number of new ones that are fertile and breed true. 



When asynapsis is complete, total sterilit}^ may be expected. Occa- 

 sionally^ there is a different result: all the chromosomes may undergo a 

 single equational division, two large spores then being formed each with 

 the hybrid's diploid complement. Rarely such a spore may function in 

 the production of polyploid offspring. 



Hybridity Involving Pol5^1oidy. — Hybrids containing different num- 

 bers of genomes may be obtained by intercrossing members of a polyploid 

 series. For example, crosses of diploid and tetraploid species may yield 

 triploid hybrids, those between tetraploids and octoploids may produce 

 hexaploid hybrids, etc. Polyploid hybrids may also be obtained by 

 inducing chromosome doubling in plants already hybrid in constitution. 

 The characters exhibited by such plants, provided they develop success- 

 fully, depend not only upon the kinds of parental genes and their inter- 

 action, but often upon the relative number of parental genomes as well. 

 Thus in maize-teosinte hyl)rids the tetraploid type with two genomes from 

 each parent resembles the diploid hybrid, whereas the triploid hj^brid with 

 one maize and two teosinte genomes looks more like teosinte. The same 

 tendenc}^ is strikingly shown in more extensive series of radish-cabbage 

 hybrids and moss hybrids. 



The breeding behavior of these allopolyploid plants depends of course 

 upon the number of genomes, the type and regularity of synapsis, the 

 viability of spores, gametes, and zygotes, the ratio of genome numbers 

 in embryo and endosperm, and other factors. The calculation of expected 

 genetical ratios becomes a complex matter, yet for certain types, notably 

 the one in which there are two genomes from each parent (see next 

 section), the expectations have in several instances been approximated by 

 latios observed in the breeding plot. 



Chromosome behavior at meiosis in polyploid hybrids is illustrated 

 in the following cases. In a fertile hexaploid hybrid poppy formed by 

 crossing the diploid Papaver nudicaule (7 chromosomes in gamete) with 

 the decaploid P. striatocarpum (35 chromosomes in gamete) the micro- 

 sporocytes showed 21 bivalents: all the chromosomes found mates. In 

 contrast to this, a hexaploid hybrid rose formed by crossing a diploid 

 form (7 in gamete) with a decaploid form (35 in gamete) showed onl}^ 7 

 bivalents, the remaming 28 chromosomes appearing as univalents. In 

 hybrids like the latter the chromosomes often show a very characteristic 

 type of subsequent behavior: the bivalents disjoin and pass poleward, 

 after which the univalents, now longitudinally double, occupy the equator 



